One of the four colours is designated as the background
colour -- one can then plot onto this in one of the other
three colours.

The "plotted" colour is selected using the Atom's "Colour"
command provided by the Floating Point ROM, and the
sets are alternated by using:

?#B002 = ?#B002:8

The default (switchon) colour set is Red, Green, Blue and
Yellow (with Green as the background in graphics modes).

1

INSTALLATION

Before installing the Atom Colour Board check visually that
it is not damaged in any way. Undo the two screws which
secure the bottom of the Atom.

It is necesary to break the existing connection between the
video amplifier and the U.H.F Modulator. This is done by
gently pushing C2 towards the rear of the ATom board
exposing a Plated Through Hole as illustrated in the dia-
gram. Cut the copper track on side 2 as shown. Remove
R11, a 3K9 resistor, from the main P.C.B.

Now remove the 6847 from the socket on the main P.C.B.
and insert it in the socket on the Atom Colour Board (en-
suring correct orientation) and then plug the entire assembly
into the vacant 6847 socket on the main board. Now solder
the flying lead from the Atom Colour Board to the positive
end od C2. Secure the Colour Board by use of two double
sided sticky pads, one on IC and the other on IC .

Before fixing the case bottom, the Colour board should be
tested. Connect the U.H.F. Modulator output to a colour
television and supply power to the Atom. Tune the television
to channel 36 and press BREAK on the Atom; the ACorn
Atom prompt should appear in green on a black background.
If no colour is obtained it may be that the sub-carrier fre-
quency is incorrect, adjust VC1 on the Colour Board with
a small srewdriver, and check that the colour controls on
the television set are correctly adjusted. If the picture will
not synchronise, slight adjustment of VR1 may be necessary;
if the prompt is not visibe but the picture is syncronised
then adust VR3.

When the above program is run the entire Atom character
set will be displayed at the top of the screen, pressing SHIFT
will alternate between the character colour sets. This is
done in line 60 (?#B002 = ?#B002:8)

3

CIRCUIT DESCRIPTION

The circuit breaks down into three basic areas as follows:

the 50 Hz converter
the R.G.B. decoder
the colour encoder.

IC2 to IC8, Q1 to Q4 and associated components form the
50 Hz converter. The 3.579MHz CLK is taken from the
main Atom P.C.B. via pin 33 of the socket. The Field Sync
signal is taken from the 6847 IC and has 25 lines added at
each end of the frame. During the addition of the 25 lines
the 6847 clock is stopped. This extends the frame to effec-
tively 213 lines of 64uS which gives a 50 Hz frame.

The luminance signal from the 6847 is added with the 25 line
inserts on Q1/2 to give Y the combined luminance signal
which, when the 6847 is stopped, consists of line sync for
25 lines.

The Y signl is fed to the comperators IC11 nd IC12. Adjust-
ment of VR3 gives CSYNC at the output of IC15 pin 3.
VR1 is adusted to give video information at IC12 pin 12.

The signals øA and øB and CHB are fed into IC9 and IC10 which
are wired as treshold detectors to give the appropriate
ABCD outputs dependant on the colour information given
by øA and øB (see the table on the following page).

The signals ABCD are decoded to give the required RGB
signals to drive the colour encoder or alternatively to drive
the RGB monitor via the RGB outputs from Q5,6,7.

4

CIRCUIT DESCRIPTION (continued)

The Decoding for R.G.B. is as follows:

FROM

TO

COLOUR

A

B

C

D

R

G

B

Green

L

L

L

L

L

H

L

Yellow

L

H

L

L

H

H

L

Blue

L

H

H

H

L

L

H

Red

H

H

L

H

H

L

L

Buff

L

H

L

H

H

H

H

Cyan

L

L

L

H

L

H

H

Magenta

H

H

H

H

H

L

H

Orange

H

H

L

L

H

L

L

Note: H = High or Logical 1L = Low or Logical ø
Buff is decoded as White
Orange is decoded as Red

IC21 and XL1 generate a signal of 17.7344MHz which is
counted in a four phase ring counter IC20 to give sub-
carrier phases of - 90°, 0°, 90° and 180°. The required
phases are selected by the RGB outputs and the relevant
amplitudes are obtained through R49 to R56. The chrom-
inence signals are added in the base of Q10. The luminence
signal is generated by Q9. Two parts of IC22 generate the
burst gate which is a pulse of 4uS after the negative going
edge of HS (line sync), and HSis fed to a section of IC7
to generate the alternate line phase switching. The first
section of IC17 is used to determine the polarity of CSYNC
to be fed to the R.G.B. monitor output.